CVE-2026-21066: libcodec2 Buffer Overflow Vulnerability

CVE-2026-21066 Overview

CVE-2026-21066 is an improper input validation vulnerability in the libcodec2_sec_flacdec.so library used in Samsung mobile devices. The flaw allows local attackers to write memory outside allocated buffer boundaries when processing crafted FLAC audio data. Samsung addressed the issue in the SMR Aug-2026 Release 1 security maintenance release. The vulnerability is categorized under [CWE-20] Improper Input Validation and requires local access with no user interaction to exploit.

Critical Impact
A local attacker can trigger out-of-bounds memory writes in the FLAC decoder, potentially corrupting adjacent memory structures and impacting confidentiality, integrity, and availability on affected Samsung devices.

Affected Products

Samsung mobile devices using libcodec2_sec_flacdec.so
Samsung firmware releases prior to SMR Aug-2026 Release 1
Samsung Android-based devices with the vulnerable FLAC decoder component

Discovery Timeline

2026-08-10 - CVE-2026-21066 published to NVD
2026-08-10 - Last updated in NVD database
SMR Aug-2026 Release 1 - Samsung releases security patch via monthly security maintenance release

Technical Details for CVE-2026-21066

Vulnerability Analysis

The vulnerability resides in libcodec2_sec_flacdec.so, a Samsung-specific shared library that handles Free Lossless Audio Codec (FLAC) decoding. The library fails to properly validate input before performing memory write operations. When the decoder processes malformed or crafted FLAC data, control-flow logic writes beyond the boundaries of an allocated buffer.

Out-of-bounds writes in native audio decoding libraries can corrupt adjacent heap structures. An attacker who controls the written data may overwrite function pointers, virtual table entries, or object metadata. This can escalate a memory corruption primitive into arbitrary code execution within the media processing context.

The attack requires local access, meaning the attacker needs a foothold on the device or must convince a user to open a malicious media file through a locally installed application. The EPSS score is 0.119% with a percentile of 2.081, indicating a low probability of imminent exploitation.

Root Cause

The root cause is missing or insufficient bounds checking on attacker-controlled fields within the FLAC bitstream. The decoder trusts size or offset values embedded in the media container without validating them against destination buffer sizes. Attackers craft FLAC files that specify values inconsistent with the library's memory allocation assumptions.

Attack Vector

Exploitation requires a local attack vector. An attacker delivers a crafted FLAC audio file to the target device through messaging, download, or a malicious application. When the vulnerable decoder parses the file, the improper input validation triggers an out-of-bounds write into process memory.

The vulnerability manifests during FLAC frame processing in the Samsung security codec library. See the Samsung Mobile Security Update for technical details on the affected components and fixed versions.

Detection Methods for CVE-2026-21066

Indicators of Compromise

Unexpected crashes or restarts of media processing services parsing FLAC files
Abnormal signal termination (SIGSEGV, SIGABRT) in processes loading libcodec2_sec_flacdec.so
Tombstone files in /data/tombstones/ referencing the FLAC decoder library
Presence of unsolicited FLAC files delivered via messaging or third-party applications

Detection Strategies

Monitor Android system logs for native crashes containing libcodec2_sec_flacdec in the backtrace
Inspect media files received from untrusted sources for malformed FLAC headers or inconsistent frame metadata
Deploy mobile threat defense agents capable of identifying anomalous behavior in media parsing processes
Correlate media file delivery events with subsequent process crashes to identify targeted exploitation attempts

Monitoring Recommendations

Enable centralized collection of Android tombstones and native crash reports from managed devices
Track firmware patch levels across the mobile fleet to identify devices missing SMR Aug-2026 Release 1
Alert on installations of unofficial applications that request access to media storage or audio processing APIs

How to Mitigate CVE-2026-21066

Immediate Actions Required

Apply the SMR Aug-2026 Release 1 update to all affected Samsung mobile devices
Verify the security patch level under Settings > About phone > Software information after updating
Restrict installation of applications from untrusted sources through mobile device management policies
Educate users to avoid opening FLAC audio files received from unknown senders

Patch Information

Samsung addressed CVE-2026-21066 in the Security Maintenance Release SMR Aug-2026 Release 1. The patch adds input validation to the FLAC decoder before performing memory operations. Refer to the Samsung Mobile Security Update bulletin for the complete list of updated components and eligible device models.

Workarounds

Disable automatic media preview features in messaging applications until the patch is applied
Block delivery of FLAC audio attachments at the mobile device management (MDM) gateway where feasible
Isolate high-value devices from untrusted networks and communication channels until patching is complete

bash

# Verify Samsung security patch level on managed devices
adb shell getprop ro.build.version.security_patch
# Expected output should reflect SMR Aug-2026 Release 1 or later